Well casing and method of making it



whose physical Patented Apr. 11, 1944 WELL CASING AND MAKING ,IT

Protin, Charleroi, and J oseph'E.

Edward J.

METHOD OF Simonin, Roscoe, Pa.

No Drawing.

Application November 15, 1941,

Serial No. 419,296

Claims.

This invention relates to well casing of steel characteristics are materially improved to resist more-effectively the stresses to which it is normally subjected.

This application is a continuation-in-part of our copending application Serial No. 302,398, filed November 1, 1939.

y In recent years-the practice has been to drill very deep oil wells to tap oil-bearing strata, and usually the depth to which a well can be drilled is determined by the strength of the casing used to line the-well. When in anoil well, casing is subjected to tremendous collapsing pressures at increased depths and it is heavily stressed longitudinally due' to the weight of the casing string in pull-out operations, and also in supporting the string in some types of wells. Heretofore, various efforts have been madeto increase the strength of well casing. These have included cold compressing plain steel casing to increase the yield point of its metal, and also resorting to the use of special alloy steels for making the casing. Increasing the wall thickness of the casing is objectionable because of increase in weight with attendant increase in. cost.

The use of special alloy steels in the manufacture of well casing is objectionable due to expense,

and, further, is often undesirable because of difficulties in welding, threading. or other operations performed on casing: during its manufacture or during its settingin-a Well.

:Injcold working a plain steel well casing to increase its strength the prevailing practice is to reducethe diameter of, the casing about four per cent by die pressure applied transversely to the outside of the casing. This method of cold working steel well casing not only requires relatively heavy and expensive machinery and the provision of a plurality of large and. costly dies for various casing sizes, but we have found that it is characterized by imparting to thecold worked easing undesirable residual hoop stresses which materially reduce the collapse strength of the casing. Furthermore, when the'metalof well casing;- is, cold Worked; in this way its yieldpoint in longitudinal tensioniis less than its yield point in transverse compression;

Apparatus for thus cold working well casing comprises. a pluralityof cooperating dies, u'sua1ly two, each formed with a casing-engaging groove of about ISO-degrees and-mounted for relativelateral reciprocation toand-from a casing plast-ically to compress it, thecasing being placed longitudinally in thesedies and being squeez ed over its-full length-"by them. We -have foun'd th-at casin treated in this manner is not uniformly cold worked throughout 'itscircumference'. The cold working is greatest in those areas of the casing at and adjacent to the lines of joinder of the ,twopart dies, and the cold working is at a minimum inthe portions of the casing at and adjacent to the valleys of the dies. Impact tests on specimens cut from the portions of such casings positioned atthe lines of 'oinder, of the diesshowinipact resistances materially below those of specimens cut from the second named portions of the casing. In other words, he pOItlOIlS Offihe' casing atand adjacent to the lines of j oindr of the dies have less ductility thanthe portions at and adjacent the valleysof the dies.

While this variationin impact resistance is in itself obj ectionalo'le, particularly if the metal is so severely cold workedin the region of'the joinder of the dies as to be objectionably' brittle, the residual stresses in the casing arising fromthe nonuniform plastic fiowof its 'etal are objectionable because they result in a distortion of the casing to such out-of-rouridness as toreduce appreciably its resistance to collapse.- Moreover, even if the cold working is circumferentially' uniform, we have found that die-pressed casing has its" inner wall fibers under residual compression stresses and its outer Wall fibers under residual tension stresses so that when a ring ofthe casing issplit the two ends of thering-spring apart showingthe presence of undesirable residual hoop stresses which not only may cause out-or-rcundne'ss but which also may add to anycollapsii'ig stresse's s'o as to cause premature collapse of the casing under pressure.

When well casinghas -been cold worked by die pressing it is often necessar subsequently to straighten it. This is usually done by cross-roll straightening apparatus which imparts further undesirable and objectionable residual stresses'to the casing, and obviously increases its cost of manufacture.

The general obj ect'of our invention is to provide a stress-freewell casing made of steel that is so cold worked and straightened that'its collapse and longitudinal tensile strengths are materially greater than those of the same metlal in an as rolled, normalized, or annealed state, the metal of the casing' having substantially the sam impact and-elongation values as in suchistatei Well casing provided according to our invention comprises a longitudinally-stressed, hollow cylindrical bodyof steel having .a substantially uniform transverse compression yield point throughout; which is appreciably higher than the the ends of the casing being gripped in circumferential com-- worked by stretching it lon- I longitudinally to reduce the outside the casing up to about two per cent,

pression, to render the casing substantially free from undesirable residual hoop stresses, and to increase its longitudinal yield point in tension above that in circumferential compression.

It is known that tubes having a ratio of diameter to wallthickness of greater than about offer a, resistance to external collapsing pressures which isv due primarily to what is generally termed egg shell effect. In such tubes the strength of the material forming them has little 'to do with their resistance to collapse. However, when the diameter to wall thickness ratio of a tube drops belowabout 30 the strength of the material forming the tube becomes important and the greater the resistance of the metal to compression the greater is the resistance to collapse of the tube. Therefore, the well casing provided according to our invention has a diameter to wall thickness ratio of less than about 30.

Various types of apparatus may be used to produce our improved cold worked oil well casing.- For example, we may use screw or fluid actuated apparatus of various kinds to stretch the casing longitudinally, but one verysatisfactory apparatus includes a pair of gripping or clamping elements of the same general structure as the clips used to support a string of casing in a well, which gripping elements are mounted so as readily to be secured to the opposite ends, of a length of well casing tobe stretched. One of the gripping elements is usually fixed and the other is mounted for movement longitudinally ofthe casing under the action of an hydraulic rain or piston. Preferably, We provide a'gauge in conjunction with the hydraulic mechanism for indicating the total force appliied by the hydraulic mechanism. Thus, we are able to stretch longitudinally thecasing so as to raise the yield point of its metal to a desired predetermined amount of its ultimate strength.

By way of example, by forming well casing of steel having from .2 to .6 carbon, .5 to 1.5 manganese .005 to .3 phosphorus, .01 to .2 sulfur, .05 to .5 silicon, and the remainder substantially iron, which steel may normally have a yield point in transverse compression of from 35,000 to 75,000 pounds per square inch and an ultimate strength of 80,000 to 120,000 pounds per square inch, we are able to raise theas-rolled, normalized, or annealed state yield point in compression byf'the stretching operation up to 70,000 to 100,000 or more pounds per square inch. The yield point of the steel can be increased by cold Work stretching up to any desired percentage of the ultimate strength, as for example, 60 to 95 percent. k V

Usually, the cross section of the casing in square'inches is determined and this figure is divided into thetotal force applied by the hy-.

malized, or annealed state of a....\ p Af tion of the casing in square inches is"5.4317 then the total load applied to each square inch of the casing is 90,100 pounds per square inch. With a steel having a yield point in an as-rolled, nor- 55,000 pounds per square inch and an ultimate strength of 100,000

pounds per square inch, subjecting the casing to a longitudinal load of 9,100 pounds per square inch will increase the yield point of the casing to approximately 90,100 pounds per square inch jwithout affecting the ultimate strength of the casing. These are yield points in longitudinal tension, and are somewhat higher than the corresponding yield points in transverse compression which are about 88 per cent of the longitudinal yield points. For supporting long lengths of casing in some typesof wells, and for pulling casing from walls, it is highly desirable to have its longitudinal tensile strength as great as possible.

From the foregoing it willbe understood that We stretch steel well casing, .formed usually of ordinary steel, under a. longitudinal force which will raise the yield points of the casing, both in transverse compression and in longitudinal tension, from its original yield points in an asrolled, normalized, or annealed state to yield points which are much higher and which may approach the ultimate strength of the casing. We have found that in longitudinally stretching casing in the manner described the outside diameter of the casing is reduced ordinarily in the neighborhood of between aboutone-half and two per cent, and usually in theneighborhood of about one per cent. The casing may originally be formed slightly over size sothat when it is stretched longitudinally its finished outside diameter will be well within prevailing standards and tolerances.

We have found that treating well casing as heretofore described not only "efiects a desired cold working of the casing steel to increase its yield point in both compression and tension to any desired values and within the ultimate strength of the steel, and does so uniformly throughout the casing, which increase materially improves the resistance of the casing to collapse and to pull, but we have found that the resulting casing possesses various noteworthy characteristics, First, the casing is accurately and rapidly straightened simultaneously with the cold working. Second, we have foundithat'the casing is circumferentially rounded in the stretching operation. Often, seamless well casing, and even casing made by other methods, such as electrically welded casing, may be slightly oval after normal manufacturing steps. The action of the stretch ing operation pulls the casing 'to circumferential roundness. Third, well casing strengthened and cold worked as herein disclosed is free of undesirable residual hoop stresses. Thus, there is no tendency for the casing to be pulled out-of-round by residual stresses, which out-of-roundness may reduceits resistance to collapse as above stated. Further, there are no residual hoop stresses present in the casing which add to externally applied collapsing forces in the use of the casing. Fourth, wehave found that the hardness and resistance annealed state. Thus, by

to impact has either not change is not appreciable or harmful. Fifth, our cold worked and straightened well casing has suffered either no decrease in per cent elongation or only such minor changes in per cent elongation have occurred as not tobe appreciable. In elongation, and the impact resistance of our cold stretched and straightened casing are substan-.

fact, the hardness, the per cent tially identical to the (hardness, the per cent elongationand impactresistance of the metal in an as-rolled, normalized, or annealed state prior to cold working it, Sixth, no change in microstructure is apparent in the cold worked and straightened casing. Seventh, no change in flattening test results is noticeable in the cold stretched and straightened casing.

As illustrative of the characteristics and advantages of our improved well casing, the following specific example is given. Four lengths of 55% inch outside diameter well casing having an average wall thickness of .330 inch were made alike in every respect of steel having the following analysis: carbon .47, manganese 1.05, phosphorus .017, sulfur .033, silicon .25, and the remainder substantially iron, Three of the lengths, namely those identified below as Nos. 1, 2 and 3, were cold worked and straightened as heretofore described. The length identified below as No. 4 was not treated after hot forming. The degree of cold working and the result of tests made of the four lengths are shown in the following table:

Pipe No.

Condition Tension load lbs. per square inch l 80, 700 84,150 90,100 None Resulting physicals:

Longitudinal tension yield lbs. per square inch 81,600 83, 500 7,700 63,400 Transverse compression yield lbs. per square inc 71,100 73, 600 78,800 58,900 Tensile lbs. per square inch 108, 600 108,700 109,200 108,200 Per cent elongation in two inches 21. 5 22. 22.0 22. 0 Oharpy impact ft. lbs... 17. 9 18.0 17. 3 18. Flattening per cent reduc tion 60. 0 63. 0 60. 0 60. 0 Collapse lbs. per square inch T, 500 7, 250 7, 200 6, 400 Internal hoop stress lbs.

per square inch None None None 9, 740 Per cent reduction in O. D .54 .81 .92 0 Brinncll hardness u, 223 223 223 223 Tension loaded. 3 Hot rolled.

Well casing provided according to our invention has materially improved physical characteristics which particularly adapt it to the lining of deep oil wells. The use of expensive multi-die apparatus for cold compressing oil well casing is eliminated. The changing of apparatus used to stretch casings of various sizes necessitates only changes in relatively inexpensive and readily handled gripping elements. Our improved well casing is not only cold stretched to improve its resistance to collapse and to increase its longitudinal strength, but is simultaneously straightened and relieved of undesirable residual hoop stresses without loss of impact resistance, hardness, and per cent elongation characteristics which in our casing are substantially identical to those in the casing in an as-rolled, normalized, or

our present invention we provide a less expensive and a particularly improved Well casing possessing needed, and avoiding undesirable characteristics.

We claim: v1. A well casing made of steel and having a ratio of diameter to wall thickness of less than about 30, the casing while cold having been .uniformly stretched longitudinally .beyond its yield ,point and thereby permanently elongated by applying tension to its ends, and consequent upon such stretching the metal of the casing being characterized by having its yield point in transverse compression materially higher than and its hardness and ductility substantially the same as in its hot worked condition, and by having its yield point in longitudinal tension greater than that in transverse compression, and the metal of the casing having uniform and like internal stresses throughout its wall thickness and as a consequence thereof being free from residual hoop stresses.

2. A well casing having a ratio of diameter to wall thickness of less than about 30 and made from steel containing from about 0.20 to 0.60 per cent carbon, from about 0.50 to 1.50 per cent manganese and from about 0.05 to 0.50 per cent silicon, the remainder being substantially iron, the casing while cold having been uniformly stretched longitudinally beyond its yield point and thereby permanently elongated by applying tension to its ends, and consequent upon such stretching the metal of the casing being characterized by having its yield point in transverse compression materially higher than and its hardness and ductility substantially the same as in its hot worked condition, and by having its yield point in longitudinal tension greater than that in transverse compression, and the metal of the easing having uniform and like internal stresses throughout its wall thickness and as a consequence thereof being free from residual hoop stresses. 1 g

3. A well casing having a ratio of diameter to wall thickness of less than about 30 and made from a steel which in its hot worked condition has a yield point in transverse compression of from about 35,000 to 75,000 pounds per square inch, the casing while cold having been uniformly stretched longitudinally beyond its yield point and thereby permanently elongated by applying tension to its ends, and consequent upon such stretching the metal of the casing being characterized by having a yield point in transverse compression of from about 70,000 to 100,000 pounds per square inch and by having its hardness and ductility substantially the same as in its hot worked condition, and by having its yield point in longitudinal tension greater than that in transverse compression, and the casing having uniform and like internal stresses throughout its wall thickness and as a consequence thereof being free from residual hoop stresses.

4. A well casing made of steel and having a ratio of diameter to wall thickness of less than about 30, the casing while cold having been uniformly stretched longitudinally beyond its yield point and thereby permanently elongated by applying tension to its ends, and consequent upon such stretching the metal of the casing being characterized by having its yield point in transverse compression materially higher than and its hardness and ductility substantially the same as in its hot worked condition, and by having its yield point in longitudinal tension greater than that in transverse compression, and consequent upon such stretching the metal of the casing having uniform and like internal stresses throughout its wall thickness and as' a consequence thereof being free from residual hoop stresses, round and straight.

5. The method of making, from hot worked steel tubing, well casing free from residual hoop stresses and having a ratio of diameter to Wall thickness of less than about 30, the step consisting of uniformly stretching the tubing longitudinally while cold beyond its yield point by apply- 'ing tension to its ends and thereby permanently elongating it and imparting to the metal thereof a yield point in transverse compression materially higher than when in its hot worked condition while maintaining the hardness and ductility of the metal thereof substantially the same as in its hot worked condition, and thereby also imparting to the metal of the stretched tubing a yield point in longitudinal tension greater than in transverse l0 compression.

EDWARD J. PROTIN. JOSEPH E. SIMONDI. 

